A multiple quantum well structure comprises a first plurality of relatively narrow bandgap semiconductor layers and a second plurality of relatively wide bandgap semiconductor layers. The relatively narrow bandgap layers are interleaved with and contiguous with the relatively wide bandgap layers. The wide-bandgap layers should exhibit a conduction and/or valence band step sufficiently large to confine electrons and/or holes respectively to the narrow bandgap layers. In other words, the steps in the conduction and valence bands serve to define quantum wells whose widths are coextensive with the narrow band gap layers. These quantum wells confine the charge carriers to the narrow bandgap layers and tend to inhibit the transverse movement of the charge carriers from one layer to the next.
Preferably, the adjacent wide and narrow bandgap layers are substantially lattice matched so that the heterojunctions formed there between are substantially defect free. The multiple quantum well structure may be formed by alternatingly depositing AlGaAs and GaAs layers on a semi-insulating GaAs substrate. The multiple quantum well structure may also be formed from other III-V compounds such as the InP-InGaAsP or InAs-GaAsSb materials systems.
Multiple quantum well structures have found numerous uses in electronics and optical-electronics. Such devices may be used as infrared radiation sources (see for example, Esaki et al. U.S. Pat. No. 4,163,238); negative resistance devices (see, for example, Esaki et al. U.S. Pat. No. 4,250,515); FET devices (see, for example, Dingle et al. U.S. Pat. No. 4,163,237); and as frequency multipliers (see my patent application, "Multiple Quantum Well Frequency Multiplier Circuit" Ser. No. 768,671, filed on Aug. 23, 1985 and assigned to the assignee hereof). Multiple quantum well devices may also be used as memory devices in which charge is stored and released from the quantum wells. As more fully described in such references, a quantum well, as we use it herein, is a region having a conductor or valence band step of sufficient magnitude to confine carriers to that region.
While quantum wells formed from alternating wide and narrow bandgap layers may be used to confine charge carriers to the narrow bandgap layers and inhibit transverse movement of charge carriers from one layer to the next, in particular semiconductor devices, it may be desirable to laterally confine charge carriers to particular regions of the narrow bandgap layers.
Heretofore, such lateral confinement has been achieved by a variety of techniques. For example, if the alternating wide and narrow bandgap regions are n-type material, then lateral confinement of electrons to particular regions of the narrow bandgap layers may be achieved by diffusing a ring-like zone of p-type dopant through the wide and narrow bandgap layers of the multiple quantum well structure. This would serve to laterally confine the electrons to the region defined by the ringlike p-type zone. Alternatively, lateral confinement of electrons to particular regions may be achieved through barriers formed by way of proton bombardment. Additionally, charge carriers may be laterally confined as result of barriers which are formed by etching away part of the multiple quantum well structure so as to expose the remaining portions of the multiple quantum well structure to the environment.
The aforedescribed techniques for lateral confinement of charge carriers in multiple quantum well structures suffer from two main shortcomings. First, the techniques are invasive. All involve damage to the narrow bandgap layers by means of diffusion of dopants, ion bombardment, or etching, etc. Such damage tends to reduce desired device properties such as charge mobility within the narrow bandgap layers.
In addition, the aforedescribed techniques for lateral charge confinement provide little or no ability to successively confine charge carriers to a particular region within the narrow bandgap layers and then deconfine the charge carriers such as by moving them to other regions within the narrow bandgap layers. The reason for this is that the lateral confinement barriers formed by etching, ion bombardment, or the diffusion of p-type dopants through the narrow bandgap layers are permanent barriers. These permanent barriers cannot be instantaneously formed so as to confine a charge packet to one region and then removed so as to laterally move the charge packet to another region. The ability to successively charge and discharge particular regions of the narrow bandgap layers of a multiple quantum well structure can be expected to be important when multiple quantum well structures are used to implement memory devices or charge coupled arrays.
Accordingly, it is an object of the present invention to provide non-invasive structures for laterally confining charge in a multiple quantum well device. Preferably, the non-invasive structures will result in the formation of non-permanent confining barriers so as to provide the device designers and users with maximal ability to laterally confine charge to particular regions within the narrow bandgap regions and to successively discharge these regions while moving the charge carriers laterally to other regions.